1. Field of the Invention
This invention relates to an hermetic pressure sensor and, more specifically, to an hermetic pressure sensor for gauge and absolute pressure sensing elements for use in environments requiring hermeticity, such as refrigeration systems, chemically hostile, expensive or poisonous pressure medias, fluids or other such environments.
2. Brief Description of the Prior Art
Sensing of pressures in environments requiring hermeticity, such as refrigeration systems, chemically hostile, expensive or poisonous pressure medias, fluids or other such environments generally requires the use of pressure sensors having walls which are impermeable to such environments and which are chemically stable, corrosion resistant and insensitive to temperature fluctuations. Such pressure sensors must hermetically seal or isolate the pressure sensing element from the pressure providing medium and prevent leakage or seepage of the pressure providing medium into the environment.
One type of sensor utilized for such pressure sensing is based upon the mechanical principle of the elastic property of materials (spring rate) wherein at least one wall of the sensor is designed and manufactured to produce predictable elastic deformation or deflection under pressure. This deflection is used to either produce electrical analog signals in the case of transducers or to mechanically open and close a set of contacts in the case of pressure switches. To accurately assess the pressures being measured, this elastic wall must be in direct contact with the pressure providing medium and must be joined to other walls of the pressure providing medium containment device in a manner which prevents or minimizes mechanical coupling of stray forces and stray deflections from such other walls into the elastic wall used for pressure measurement.
Conventionally, the elastic wall in such pressure sensors is formed utilizing a diaphragm which provides a type of hermeticity and is produced by using flexible man-made materials, such as rubber sheet, Kapton, silicone rubber, silicone gels and the like. The application of such prior art pressure sensors often requires that hermeticity of the pressure applying system be maintained for extended periods. Low cost sensors, in general, do not meet this requirement because the pressure seals are made from organic compounds as noted above. These compounds may degrade over a long time period, and/or may degrade from repeated movement thereof and/or may not be suitable for use in conjunction with certain chemical pressure fluids and/or may not be useful over the desired or required range of temperatures.
Other approaches, such as provided in the Klixon 20PS pressure switch sold by Texas Instruments Incorporated, obtain their hermeticity through welding or brazing of single or multiple flexible membranes (pressure discs) to a flanged, rigid pressure fitting at the outer perimeter of the disc. Undesired interaction between the welding process (weld solidification stress) material composition and the precalibrated membrane results in low precision sensors which exhibit sensitivity to temperature and early fatigue of the weld joint.
Bellows type sensors use corrugated (highly flexible) tubes or membranes which are pre-loaded or supported by force-carrying members to limit deflection. The bellows is brazed or welded to a flanged pressure port. Corrugation provides lowest resistance to deflection and a relatively linear (not snap acting) deflection to force ratio (spring rate) which is designed to compromise between measuring accuracy and maximum permissible pressure at the pressure port. For high system pressures, the membranes must be of such thickness as not to cause permanent deformation of the bellows membrane. The bellows primarily fulfills the function of sealing the fluid medium and also has to withstand and carry a high portion of the forces exerted by the fluid pressure.
It is therefore apparent that an hermetic sealing member is required which can be used in environments of the type described hereinabove, which provide the required properties noted hereinabove and which can also be produced relatively economically.